During certain semiconductor assembly processes, semiconductor dice are placed on a carrier such as a lead frame substrate. Electrical connections in the form of wire bonds are then made between the semiconductor dice and lead frame substrate, between individual dice, or between different points on the lead frame substrate. Gold, aluminum, silver, or copper wires are commonly used to make these wire bonds.
Wire bonds are formed at bonding sites where the electrical connections are to be made. Typically an ultrasonic transducer is used to generate ultrasonic energy to attach a free air ball (FAB) from a capillary to a semiconductor die or a carrier. This forms a ball bond at a first bonding site. Thereafter, the capillary is moved to a second bonding site as the bonding wire is fed out from the capillary, forming a wire loop. A stitch bond is then formed at the second bonding site, which is usually on the carrier and adjacent a perimeter of the semiconductor die. This completes the formation of a wire bond to electrically connect the first bonding site to the second bonding site. After all the necessary wire bonds are made, the dice, wire loops, and carrier are encapsulated with a resin material to protect them, whereby a semiconductor package is produced.
There is a continuing desire in the semiconductor industry to develop smaller semiconductor packages. Since the wire loops should be fully encapsulated in the final semiconductor package and the wire loops bonding sites are usually located adjacent the peripheries of the semiconductor dice, the surface area occupied by the package would be affected by the locations of the wire loops bonding sites. This means that the nearer the wire loops bonding sites are to the perimeters of the dice, the smaller the final semiconductor package can be.
Although it is desirable to position the wire loop bonding sites nearer to the perimeters of the dice, the ability to do so during the design of the semiconductor package is limited by various factors. One limiting factor would be the capillary size. The size of the capillary which holds and dispenses the bonding wire during wire bonding would physically limit how close the wire loop bonding sites can be to the perimeters of the dice. Another limiting factor is the weakness in the neck of the bonding wire at the point where the bonding wire is bent towards the second bonding site. The nearer the wire loops bonding sites are to the perimeters of the dice, the more the bonding wire forming the wire loop would have to be bent towards the second bonding site, causing an increased risk that the bonding wire will crack at the neck. If the bonding wire cracks, the resulting electrical connection would become unreliable or unstable.
FIGS. 1A and 1B are side and top views respectively of a conventional wire loop 10 employed in an over-die wire bonding application. The wire loop 10 and a semiconductor die 20 are formed on a substrate 30. The wire loop 10 is formed over the top surface of the semiconductor die 20.
The wire loop 10 is bonded at a first bonding point 32 on the substrate 30. The wire loop 10 extends therefrom substantially vertically and substantially parallel to a first side of the semiconductor die 20, and bends at a first kink 12 towards a second bonding point 34. A span portion 16 of the wire loop 10 extends from the first kink 12 substantially horizontally and parallel to the top surface of the semiconductor die 20, and bends at a second kink 14 towards the second bonding point 34. A slope portion 18 of the wire loop 10 begins from the second kink 12 at an opposing second side of the semiconductor die 20, and inclines towards the second bonding point 34 where it is bonded to the substrate 30. The wire loop 10 is substantially located on a vertical plane. In other words, the first bonding point 32, the first kink 12, the span portion 16, the second kink 14, and the second bonding point 34 are all located substantially on the same vertical plane.
A side view horizontal span length is defined as the horizontal distance substantially parallel to a width of the semiconductor die 20 between the first kink 12 and the second kink 14, as viewed from the side view shown in FIG. 1A. A side view horizontal slope length is defined as the horizontal distance between the second kink 14 and the second bonding point 34, as viewed from the side view shown in FIG. 1A. It is usually more meaningful to express the side view horizontal span length and the side view horizontal slope length as percentages of a side view total horizontal distance (comprising a sum of both lengths). A second kink vertical height is the vertical distance between the second kink 14 and the second bonding point 34, as viewed from the side view shown in FIG. 1A. A side view vertical landing angle is the angle at the second bonding point 34 formed between the surface of the substrate 30 or the second bonding point 34 and the slope portion 18, as viewed from the side view shown in FIG. 1A.
It is desirable for the first and second bonding points 32, 34 to be as close to the respective sides of the semiconductor die 20 as possible, in order for the packages to be smaller. Hence, it should be appreciated that the side view horizontal span length percentage should be as high as possible, preferably above 85% of the side view total horizontal distance, and that the side view vertical landing angle should be high as possible, preferably above 80 degrees. However, factors such as the shape and size of the capillary which holds and dispenses the bonding wire during wire bonding would physically limit the side view vertical landing angle of the wire loop 10 to below 80 degrees and limit the side view horizontal span length to be below 80% of the side view total horizontal distance.
In addition, the second kink vertical height should be sufficiently high in order to provide an adequate clearance from the semiconductor die 20 surface. This is to avoid the wire loop 10 contacting the semiconductor die 20, and causing a short circuit. It should be noted that by increasing the second kink vertical height, the side view vertical landing angle would also increase (assuming the side view horizontal span length and the side view horizontal slope length are kept constant).